Grid-connected converter control method and system with both fault current limiting and transient stability improvement

Abstract

The present invention discloses a control method and system for a grid-type converter that combines fault current limiting and transient stability improvement. The method mainly achieves fault current limiting and transient stability improvement from two aspects. First, by switching the converter output voltage involved in the active power calculation to a virtual internal potential, the equivalent virtual resistance is transferred from the converter side to the grid side, indirectly enhancing the resistive component of the grid impedance, thereby improving the transient synchronization stability of the grid-type converter under a grid short-circuit fault; second, the virtual impedance is adaptively adjusted according to the severity of the fault, so that the grid-type converter outputs the maximum short-circuit current within its overcurrent limit range, while ensuring the safe operation of the converter, providing power support to the grid as much as possible. This method realizes the independent management of virtual resistance and virtual reactance, overcoming the problem that the existing grid-type converter fault current limiting control method based on virtual impedance is prone to cause transient instability of the device.

Description

Technical Field

[0001] The present invention relates to the field of network control technology, and in particular to a network-type converter control method and system with both fault current limiting and transient stability improvement. Background Art

[0002] With the increasing penetration of renewable energy, new power systems are gradually transitioning from being dominated by synchronous generators to being dominated by power electronic converters. This results in reduced system inertia and decreased operational stability. In recent years, grid control technology has garnered significant industry attention as a viable approach to addressing this power system transformation. Through grid control technology, converters can simulate the inertia and damping of traditional synchronous generators and externally represent the operating characteristics of the voltage source, thereby improving the frequency stability of new power systems.

[0003] However, grid-type converters are composed of semiconductor switching devices, and their short-term overcurrent capability is only 1.2 to 2.0 times the rated current. In the event of a grid short-circuit fault, they will trip, shut down, or even burn out. Therefore, fault current limiting measures are necessary. Virtual impedance control is a commonly used current limiting measure that maintains the grid-type characteristics of the converter during grid short-circuit faults without adding additional hardware costs, leading to its widespread application. However, this method places high demands on the value of the virtual impedance. If the virtual impedance is too small, the fault current will still exceed the limit, failing to meet the converter's safety requirements. Conversely, if the virtual impedance is too large, transient synchronous instability will occur in the converter, making it difficult to ensure converter stability.

[0004] In the event of a grid short-circuit fault, the introduction of virtual impedance leads to a conflict between transient stability and fault current limiting. Therefore, an improved control method that can effectively achieve both transient synchronous stability and fault current limiting capabilities of grid-connected converters is urgently needed. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a grid-type converter control method and system that combines fault current limiting and transient stability improvement in response to the shortcomings of the existing technology. Under the premise of suppressing transient overcurrent and steady-state fault current peak, it maximizes the active support capability of the faulty power grid, and effectively improves the transient synchronization stability of the grid-type converter under large disturbances.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a control method for a grid-type converter with both fault current limiting and transient stability improvement, which is suitable for improving the transient stability, current amplitude limitation and fault current support capability of the grid-type converter under a short-circuit fault in the power grid. The topology of the grid-type converter is an active neutral-point-clamped (ANPC) three-level structure. The DC side of the converter is connected to a DC source (energy storage equipment or renewable energy power generation unit), and the AC side of the converter is connected to an L filter; the L filter is connected to a common coupling point (PCC) through a grid-connected switch to achieve connection to the power grid. The control method of the grid-type converter includes the following steps:

[0007] 1) At the start of each sampling period, the sampling unit measures the terminal voltage U of the grid-type converter. o , grid-connected current I g Sampling and low-pass filtering are performed; U o =[u oa ,u ob ,u oc ]; I g =[i ga ,i gb ,i gc ];u oa ,u ob ,u oc is the three-phase terminal voltage of the grid-type converter, i ga ,i gb ,i gc is the three-phase grid-connected current of the grid-type converter;

[0008] 2) The terminal voltage U obtained by sampling in step 1) is o , grid-connected current I g The virtual internal potential reference instruction of the previous sampling period is sent to the power calculation module of the control unit to obtain the output power of the actual PCC point and the virtual power of the virtual PCC point. The calculation formula is:

[0009]

[0010] Where, E vref is the virtual internal potential reference instruction of the current sampling period, E vref =[e vrefa ,e vrefb ,e vrefc ], e vrefa ,e vrefb ,e vrefc is the three-phase virtual internal potential instruction of the grid-type converter; e is a natural constant; s is the Laplace operator; Tsw is the sampling period of the grid-type converter; P e is the output active power of the grid-type converter; Q e is the output reactive power of the grid-type converter; P vir is the virtual active power of the grid-type converter; T is the transpose operator of the vector or matrix;

[0011] The method of the present invention only needs to switch to the virtual internal potential to participate in the calculation of the active power loop feedback quantity under fault conditions. Under the default first sampling period, when the grid-type converter works normally, any value of the internal potential reference instruction will not affect the actual operation of the project, so the U value of the current sampling period can be taken. o In the present invention, E vref is the virtual internal potential reference instruction of the current sampling period, E vref ·e -sTsw Indicates the virtual internal potential reference instruction of the previous sampling period.

[0012] 3) The terminal voltage U o , grid-connected current I g The data is sent to the adaptive virtual impedance control module to obtain the minimum amplitude U of the three-phase terminal voltage in real time. omin , steady-state virtual impedance amplitude Z v , transient virtual impedance amplitude R tvi And the steady-state virtual impedance control enable flag σ svi ;

[0013] 4) The output active power P of the grid-type converter e , output reactive power Q e , virtual active power P vir and flag σ svi Sent to the improved control link of the virtual synchronous generator, the angular frequency deviation Δω and the internal potential amplitude instruction E are obtained. vref , the calculation formula is

[0014]

[0015] Where, J is the moment of inertia; D p is the damping coefficient; D q is the reactive-voltage coefficient; E n is the rated voltage amplitude; P ref is the active power instruction; Q ref is the reactive power instruction; — is the negation operator, when σ svi When it is 0, the control object is P e , when σ svi When it is 1, the control object is P vir ;

[0016] 5) Using the angular frequency deviation Δω, calculate the internal potential phase angle instruction θ vref ;

[0017] 6) Set the internal potential phase angle command θ vref and internal potential amplitude command E vref Send it to the dq inverse transformation module to obtain the virtual internal potential reference instruction E of this sampling period vref ;

[0018] 7) Using the terminal voltage amplitude U omin , angular frequency deviation Δω and flag σ svi , generating a virtual impedance angle

[0019] 8) Using the steady-state virtual impedance amplitude Z v , transient virtual impedance amplitude R tvi and virtual impedance angle Get the virtual resistance R v and virtual reactance X v , the calculation formula is

[0020]

[0021] 9) The grid-connected current I g , virtual resistance R v and virtual reactance X v It is sent to the impedance voltage drop generation module to obtain the virtual impedance voltage drop, and combined with the virtual internal potential reference instruction E of the current cycle vref , and obtain the modulation voltage E of the grid-type converter ref , the calculation formula is

[0022]

[0023] 10) Modulate the voltage E ref Perform SPWM modulation, output PWM modulation signal, and control the switching state of the power devices of each phase bridge arm in the grid-type converter.

[0024] Furthermore, the virtual impedance angle in the present invention The acquisition process includes:

[0025] 1) The terminal voltage amplitude U omin Send it to the virtual impedance angle generation module to obtain the initial value of the virtual impedance angle The calculation formula is

[0026]

[0027] 2) Determine the flag bit σ svi state, if σ sviIf the angular frequency deviation Δω is 1, the angular frequency deviation Δω is fed forward to the virtual impedance improvement control through the PI regulator to dynamically adjust the virtual impedance angle. If σ svi is 0, the input of the PI regulator is 0; the calculation formula of the virtual impedance angle is

[0028]

[0029] Where k pzv is the proportional coefficient of the PI regulator; k izv is the integral coefficient of the PI regulator.

[0030] In the present invention, the adaptive virtual impedance control includes two parts: transient virtual impedance control and steady-state virtual impedance control. Transient virtual impedance is used to suppress transient impact currents at the moment of fault occurrence and recovery. Steady-state virtual impedance is used to maintain the fault current amplitude at 1.2 times the rated level during the fault, preventing it from exceeding the safe overcurrent limit of the converter, while providing active support for the power grid to the greatest extent.

[0031] Furthermore, the transient virtual impedance amplitude R tvi The acquisition process includes:

[0032] 1) The terminal voltage amplitude of the last sampling period And the terminal voltage amplitude U of this sampling period omin The difference is made and its absolute value is taken as the input of fault detection. The criterion for fault detection is:

[0033]

[0034] 2) After the terminal voltage amplitude meets the fault detection criterion in step 1), the transient virtual impedance control enable flag σ output by the SR latch is tvi Flips from 0 to 1, thereby activating transient virtual impedance control; the counter starts from 0 and increases by 1 every sampling period;

[0035] 3) According to the storage value N of the counter in the current sampling period ctr , calculate the exponential decay factor G, the calculation formula is

[0036]

[0037] Where λ is the attenuation coefficient, which is used to adjust the attenuation rate of the transient virtual impedance;

[0038] 4) Using the maximum grid-connected current amplitude I gmax and exponential decay factor G, through a high-pass filter, to obtain the transient virtual impedance amplitude R tvi , the calculation formula is

[0039]

[0040] Where γ is the gain constant of the high-pass filter; β is the cutoff frequency of the high-pass filter; Z v_limit The upper limit of transient virtual impedance can be 1 to 2 times of rated impedance;

[0041] 5) The counter stores the value N in the current sampling period ctr Increase to the upper limit N max When the SR latch outputs the flag bit σ tvi Flips from 1 to 0, thereby freezing the transient virtual impedance control; at the same time, the counter stores the value N ctr Cleared.

[0042] Compared with the existing technology, the present invention has the following beneficial effects: by switching the output voltage of the converter involved in the active power calculation to a virtual internal potential under a grid fault, the present invention equates the virtual resistance to the grid side, indirectly enhancing the grid resistance and improving the transient synchronization stability of the grid-forming converter; in addition, the present invention adaptively adjusts the virtual impedance according to the severity of the fault, effectively suppressing transient overcurrent and steady-state fault current peaks, and continuously supplies 1.2 times the rated short-circuit current to the grid, supporting the grid to the greatest extent within the converter overcurrent limit, and ensuring the operational safety and active support capability of the converter equipment under grid faults. The method proposed in the present invention can maintain the grid-forming operation characteristics of the converter under a grid short-circuit fault, and does not require the prior acquisition of time-varying line impedance information. It can be extended to various voltage source power electronic equipment in various application scenarios to enhance the transient stability and fault current limiting capability of the equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the structure of a grid-type converter according to an embodiment of the present invention;

[0044] FIG2( a ) is a control block diagram of a method for controlling a grid-type converter with both fault current limiting and transient stability improvement according to an embodiment of the present invention; FIG2( b ) is an equivalent circuit diagram of a method for controlling a grid-type converter with both fault current limiting and transient stability improvement according to an embodiment of the present invention;

[0045] Figure 3 Schematic diagram of adaptive virtual impedance control of a grid-type converter according to an embodiment of the present invention;

[0046] Figure 4 A schematic diagram of the working logic of transient virtual impedance control according to an embodiment of the present invention;

[0047] FIG5( a ) is a simulation waveform of a grid-type converter according to an embodiment of the present invention using a traditional virtual synchronous generator control method when the grid voltage drops symmetrically to 0.7 pu in three phases, where A11 is the grid voltage U g Waveform, A12 is the grid-connected current I g Waveform, A13 is the virtual power angle δ vir Waveform, A14 is active power P e Waveform; Figure 5 (b) is a simulation waveform of the control method proposed by the present invention under the same fault conditions as Figure 5 (a), where A21 is the grid voltage U g Waveform, A22 is the grid-connected current I g Waveform, A23 is the virtual power angle δ vir Waveform, A24 is active power P e waveform;

[0048] FIG6( a ) is a simulation waveform of a grid-type converter according to an embodiment of the present invention using a traditional virtual synchronous generator control method when the grid voltage drops symmetrically to 0.1 pu in three phases, where A31 is the grid voltage U g Waveform, A32 is the grid-connected current I g Waveform, A33 is the virtual power angle δ vir Waveform, A34 is active power P e Waveform; Figure 6 (b) is a simulation waveform of the control method proposed in the present invention under the same fault conditions as Figure 6 (a), where A41 is the grid voltage U g Waveform, A42 is the grid-connected current I g Waveform, A43 is the virtual power angle δ vir Waveform, A44 is active power P e waveform. DETAILED DESCRIPTION

[0049] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0050] Figure 1This is a structural diagram of a grid-type converter according to an embodiment of the present invention, comprising a DC source, a DC capacitor, an ANPC three-level converter, an AC filter inductor, and a control system. The control system includes a sampling unit, a control unit, and a drive protection unit. The DC source is connected in parallel to both sides of the DC capacitor. The DC capacitor is connected to the ANPC three-level converter. The ANPC three-level converter is connected to the AC filter inductor. The AC filter inductor is connected to the actual PCC point via a grid-connected switch to achieve connection to the grid. The sampling unit is connected to the control unit. The control unit is connected to the drive protection unit. The drive protection unit is connected to the ANPC three-level converter. The control unit is mainly composed of a virtual synchronous generator improvement control link and a virtual impedance improvement control link.

[0051] See Figure 2(a), which shows a control block diagram of a grid-type converter control method that combines fault current limiting and transient stability improvement according to an embodiment of the present invention. The grid-type converter can be equivalent to a circuit model consisting of an ideal controlled voltage source and a virtual impedance in series. The controlled voltage source is connected to the virtual impedance via a virtual PCC point, and the power transmitted by the virtual PCC point is virtual active power and virtual reactive power. The virtual synchronous generator improvement control link is responsible for the grid-type converter control. Under normal operating conditions, it simulates the operating characteristics of a synchronous generator and participates in frequency and voltage regulation of the power grid. In the event of a grid short-circuit fault, the converter output voltage used in active power calculation is switched to a virtual internal potential, converting the feedback quantity into virtual active power. This transfers the equivalent virtual resistance from the converter side to the grid side, enhancing the transient stability of the grid-type converter. The virtual impedance improvement control link automatically adjusts the virtual impedance based on the fault severity to suppress the fault current amplitude. It works in conjunction with the virtual synchronous generator improvement control link and specifically includes a virtual impedance angle generation module, an adaptive virtual impedance control module, a virtual impedance calculation module, and an impedance voltage drop generation module. Referring to FIG2(b), there is shown an equivalent circuit of a control method for a grid-type converter with both fault current limiting and transient stability improvement according to an embodiment of the present invention. According to the type of power transmission, the equivalent circuit can be decomposed into a P circuit that transmits only active power and a Q circuit that transmits only reactive power. In the P circuit, the resistance between the virtual PCC point and the actual PCC point is 0, so that the transmission point of the virtual active power is moved from the virtual PCC point to the actual PCC point, and then the virtual resistance is equivalent to the grid side, achieving the same effect of the grid resistance enhancing transient synchronization stability. In the Q circuit, the virtual reactance is still located between the virtual PCC point and the actual PCC point, so the transmission point of the virtual reactive power is still at the virtual PCC point. Through the control method of the present invention, the virtual resistance and virtual reactance are independently managed to achieve the simultaneous improvement of the transient stability and safety support level of the grid-type converter. The specific method includes:

[0052] 1) At the start of each sampling period, the sampling unit measures the terminal voltage U of the grid-type converter.o , grid-connected current I g Sampling and low-pass filtering are performed; U o =[u oa ,u ob ,u oc ]; I g =[i ga ,i gb ,i gc ];

[0053] 2) The terminal voltage U obtained by sampling in step 1) is o , grid-connected current I g The virtual internal potential reference instruction of the previous sampling period is sent to the power calculation module of the control unit to obtain the output power of the actual PCC point and the virtual power of the virtual PCC point. The calculation formula is:

[0054]

[0055] Where, E vref is the virtual internal potential reference instruction of the current sampling period, E vref =[e vrefa ,e vrefb ,e vrefc ]; e is a natural constant; s is the Laplace operator; T sw is the sampling period of the grid-type converter; P e is the output active power of the grid-type converter; Q e is the output reactive power of the grid-type converter; P vir is the virtual active power of the grid-type converter; T is the transpose operator of the vector or matrix;

[0056] 3) The terminal voltage U o , grid-connected current I g The data is sent to the adaptive virtual impedance control module to obtain the minimum amplitude U of the three-phase terminal voltage in real time. omin , steady-state virtual impedance amplitude Z v , transient virtual impedance amplitude R tvi And the steady-state virtual impedance control enable flag σ svi ;

[0057] 4) The output active power P of the grid-type converter e , output reactive power Q e , virtual active power P vir and flag σ svi Sent to the improved control link of the virtual synchronous generator, the angular frequency deviation Δω and the internal potential amplitude instruction E are obtained. vref , the calculation formula is

[0058]

[0059] Where, J is the moment of inertia; D p is the damping coefficient; D q is the reactive-voltage coefficient; E n is the rated voltage amplitude; P ref is the active power instruction; Q ref is the reactive power instruction; — is the negation operator, when σ svi When it is 0, the control object is P e , when σ svi When it is 1, the control object is P vir ;

[0060] 5) Using the angular frequency deviation Δω, the internal potential phase angle instruction θ is obtained vref , the calculation formula is

[0061]

[0062] 6) Set the internal potential phase angle command θ vref and internal potential amplitude command E vref Send it to the dq inverse transformation module to obtain the virtual internal potential reference instruction E of this sampling period vref , the calculation formula is

[0063]

[0064] 7) Using the terminal voltage amplitude U omin , angular frequency deviation Δω and flag σ svi , generating a virtual impedance angle

[0065] 8) Using the steady-state virtual impedance amplitude Z v , transient virtual impedance amplitude R tvi and virtual impedance angle Get the virtual resistance R v and virtual reactance X v , the calculation formula is

[0066]

[0067] 9) The grid-connected current I g , virtual resistance R v and virtual reactance X v It is sent to the impedance voltage drop generation module to obtain the virtual impedance voltage drop, and combined with the virtual internal potential reference instruction E of the current cycle vref , and obtain the modulation voltage E of the grid-type converter ref , the calculation formula is

[0068]

[0069] 10) Modulate the voltage E ref Perform SPWM modulation, output PWM modulation signal, and control the switching state of the power devices of each phase bridge arm in the grid-type converter.

[0070] Furthermore, the virtual impedance angle in the present invention The acquisition process includes:

[0071] 1) The terminal voltage amplitude U omin Send it to the virtual impedance angle generation module to obtain the initial value of the virtual impedance angle The calculation formula is

[0072]

[0073] 2) Determine the flag bit σ svi state, if σ svi If the angular frequency deviation Δω is 1, the angular frequency deviation Δω is fed forward to the virtual impedance improvement control through the PI regulator to dynamically adjust the virtual impedance angle. If σ svi is 0, the input of the PI regulator is 0; the calculation formula of the virtual impedance angle is

[0074]

[0075] Where k pzv is the proportional coefficient of the PI regulator; k izv is the integral coefficient of the PI regulator.

[0076] See attached Figure 3 , is a schematic diagram of adaptive virtual impedance control of a grid-type converter according to an embodiment of the present invention, comprising transient virtual impedance control and steady-state virtual impedance control (see invention patent application "A Fault Current Limiting Method for a Grid-Type Energy Storage Converter Based on Dynamic Virtual Impedance", CN202311775830.7). Transient virtual impedance is used to suppress transient surge currents at the moment of fault occurrence and recovery; steady-state virtual impedance is used to maintain the fault current amplitude at 1.2 times the rated level during the duration of the fault, ensuring that it does not exceed the safe overcurrent limit of the converter, while providing active support to the power grid to the greatest extent possible. The method comprises the following steps:

[0077] 1) The grid-connected current I g Perform maximum value calculation to obtain the maximum value of the three-phase grid-connected current amplitude I gmax ;

[0078] 2) Using the terminal voltage amplitude U omin , grid-connected current amplitude I gmax, the state of the grid-type converter is evaluated to obtain the current state quantity State, which is determined based on

[0079]

[0080] Where, I n is the rated current of the grid-type converter; Z n is the rated impedance of the grid-type converter; ∩ is the intersection operator; ∪ is the union operator;

[0081] 3) According to the current state quantity State in step 2), calculate the steady-state virtual impedance amplitude Z v , the calculation formula is

[0082]

[0083] Where k rzv is the integral coefficient of steady-state virtual impedance control;

[0084] 4) Perform Boolean data conversion on the current state quantity State to obtain the steady-state virtual impedance control enable flag σ svi , mode switching for activating steady-state virtual impedance control and improved control of virtual synchronous generators.

[0085] See attached Figure 4 , is a working logic diagram of transient virtual impedance control according to an embodiment of the present invention, wherein the transient virtual impedance amplitude R tvi The acquisition process includes:

[0086] 1) The terminal voltage amplitude of the last sampling period And the terminal voltage amplitude U of this sampling period omin The difference is made and its absolute value is taken as the input of fault detection. The criterion for fault detection is:

[0087]

[0088] 2) After the terminal voltage amplitude meets the fault detection criterion in step 1), the transient virtual impedance control enable flag σ output by the SR latch is tvi Flips from 0 to 1, thereby activating transient virtual impedance control; the counter starts from 0 and increases by 1 every sampling period;

[0089] 3) According to the storage value N of the counter in the current sampling period ctr , calculate the exponential decay factor G, the calculation formula is

[0090]

[0091] Where λ is the attenuation coefficient, which is used to adjust the length of the attenuation time;

[0092] 4) Using the maximum grid-connected current amplitude I gmax and exponential decay factor G, through a high-pass filter, to obtain the transient virtual impedance amplitude R tvi , the calculation formula is

[0093]

[0094] Where γ is the gain constant of the high-pass filter; β is the cutoff frequency of the high-pass filter; Z v_limit The upper limit of transient virtual impedance can be 1 to 2 times of rated impedance;

[0095] 5) The counter stores the value N in the current sampling period ctr Increase to the upper limit N max When the SR latch outputs the flag bit σ tvi Flip from 1 to 0, freeze the transient virtual impedance control; at the same time, the counter stores the value N ctr Cleared.

[0096] The effectiveness and advancement of the control method proposed in the embodiment of the present invention are verified by MATLAB / Simulink software.

[0097] The rated capacity of the grid-type converter is 45kVA, the voltage level is DC 800V / AC 380V, the AC filter inductance is 0.5mH, the DC capacitance is 2115μF, and before the fault, the converter outputs active power 45kW and reactive power 0kVar; the grid voltage is AC 380V / 50Hz, and the grid impedance is 0.12+j0.63Ω.

[0098] 5(a) and 5(b), which show the grid voltage U under a mild symmetrical fault with a voltage drop depth of 0.3 pu after the grid-connected converter according to the embodiment of the present invention adopts the traditional virtual synchronous generator control and the proposed control method respectively. g , grid-connected current I g , virtual power angle δ vir , active power P e In Figure 5(a), a fault occurs in the power grid at 4s, and the fault current amplitude exceeds the safety overcurrent limit of 1.2I under the traditional control method. n , while the converter's virtual power angle δ vir In Figure 5(b), through the control method proposed by the present invention, the fault current amplitude is limited to 1.2I after the fault. nThis ensures the safety of the grid-connected converter and provides maximum active support for the power grid. In addition, under fault conditions, the grid-connected converter always remains synchronized with the power grid, and the transient synchronization stability is improved.

[0099] 6(a) and 6(b), which show the grid voltage U under a severe symmetrical fault with a voltage drop depth of 0.9 pu in the grid, after the grid-connected converter according to the embodiment of the present invention adopts the traditional virtual synchronous generator control and the proposed control method respectively. g , grid-connected current I g , virtual power angle δ vir , active power P e In Figure 6(a), the grid fault occurs at 2s and the fault is restored at 6.5s. Under this simulation condition, fault overcurrent and transient instability occur simultaneously. In particular, overcurrent spikes appear at the moment of fault, which greatly threatens the safety and stability of the grid-connected converter. In Figure 6(b), both the fault steady-state current and transient current spikes are suppressed, and the virtual power angle δ vir The control method proposed in the embodiment of the present invention maintains convergence throughout the fault process, and simultaneously improves the transient synchronization stability and fault current limiting capability of the grid-type converter, overcoming the problem that the existing virtual impedance-based fault current limiting control method easily leads to transient synchronization instability of the grid-type converter.

[0100] Although the preferred embodiments of the present application have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present application.

[0101] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. A control method for a grid-type converter with both fault current limiting and transient stability improvement, wherein the topology of the grid-type converter is an ANPC three-level structure; the DC side of the grid-type converter is connected to a DC source, and the AC side of the grid-type converter is connected to an L filter; the L filter is connected to a common coupling point through a grid-connected switch and is connected to the power grid; characterized in that: The following steps are involved: S1, at the starting point of each sampling period, the terminal voltage U o , grid-connected current I g Sampling and low-pass filtering are performed; U o =[u oa ,u ob ,u oc ]; I g =[i ga ,i gb ,i gc ];u oa ,u ob ,u oc is the three-phase terminal voltage of the grid-type converter, i ga ,i gb ,i gc is the three-phase grid-connected current of the grid-type converter; S2. Calculate the output power of the actual PCC point and the virtual power of the virtual PCC point using the following formula: Among them, E vref is the virtual internal potential reference instruction of the current sampling period, E vref =[e vrefa ,e vrefb ,e vrefc ], e vrefa ,e vrefb ,e vrefc is the three-phase virtual internal potential instruction of the grid-type converter; e is a natural constant, s is the Laplace operator, T sw is the sampling period of the grid-type converter, P e is the output active power of the grid-type converter, Q e is the output reactive power of the grid-type converter, P vir is the virtual active power of the grid-type converter, T is the transpose operator of the vector or matrix; Using the terminal voltage U o , grid-connected current I g Get the minimum amplitude U of the three-phase terminal voltage in real time omin , steady-state virtual impedance amplitude Z v , transient virtual impedance amplitude R tvi And the steady-state virtual impedance control enable flag σ svi ; S3, calculate the angular frequency deviation Δω and the internal potential amplitude instruction E vref , the calculation formula is: Where J is the moment of inertia, D p is the damping coefficient, D q is the reactive-voltage coefficient, ω n is the rated angular frequency, E n is the rated voltage amplitude, P ref is the active power instruction, Q ref is the reactive power instruction, — is the negation operator, when σ svi When it is 0, the control object is P e , when σ svi When it is 1, the control object is P vir ; S4. Calculate the internal potential phase angle instruction θ using the angular frequency deviation Δω. vref ; S5, the internal potential phase angle instruction θ vref and internal potential amplitude command E vref Perform dq inverse transformation to obtain the virtual internal potential reference instruction of the current sampling period; use the minimum amplitude U of the three-phase terminal voltage omin , angular frequency deviation Δω and flag σ svi , generating a virtual impedance angle S6, using the steady-state virtual impedance amplitude Z v , transient virtual impedance amplitude R tvi and virtual impedance angle Get the virtual resistance R v and virtual reactance X v : S7, using the grid-connected current I g , virtual resistance R v and virtual reactance X v Calculate the virtual impedance voltage drop and combine it with the virtual internal potential reference instruction E of the current cycle vref , and obtain the modulation voltage E of the grid-type converter ref : S8, the modulation voltage E ref Perform SPWM modulation, output PWM modulation signal, and control the on and off of the power devices of each phase bridge arm in the grid-type converter.

2. The grid-connected converter control method with both fault current limiting and transient stability improvement according to claim 1, characterized in that: The virtual impedance angle The acquisition process includes: Using the minimum amplitude U of the three-phase terminal voltage omin Get the initial value of the virtual impedance angle The calculation formula is Judgment flag σ svi state, if σ svi is 1, the input of the PI regulator is the angular frequency deviation Δω; if σ svi If it is 0, the input of the PI regulator is 0; the calculation formula of the virtual impedance angle is: Among them, k pzv is the proportional coefficient of the PI regulator; k izv is the integral coefficient of the PI regulator.

3. The grid-connected converter control method with both fault current limiting and transient stability improvement according to claim 1, characterized in that: The transient virtual impedance amplitude R tvi The calculation process includes: 1) The minimum amplitude of the three-phase terminal voltage in the last sampling period and the minimum amplitude U of the three-phase terminal voltage in the current sampling period omin The absolute value of the difference is used for fault detection. The fault detection criterion is: 2) After the minimum amplitude of the three-phase terminal voltage meets the fault detection criterion in step 1), the transient virtual impedance control enable flag σ output by the SR latch is tvi Flip from 0 to 1, the counter starts from 0 and increases by 1 every sampling period; 3) According to the storage value N of the counter in the current sampling period ctr , calculate the exponential decay factor G, the calculation formula is: λ is the attenuation coefficient; 4) Set the maximum grid current amplitude I gmax and the exponential decay factor G through a high-pass filter to obtain the transient virtual impedance amplitude R tvi : Among them, γ is the gain constant of the high-pass filter, β is the cutoff frequency of the high-pass filter, and Z v_limit is the upper limit of transient virtual impedance; 5) The counter stores the value N in the current sampling period ctr Increase to the upper limit N max When the SR latch outputs the flag bit σ tvi Flips from 1 to 0, and the counter stores the value N ctr Cleared.

4. A grid-type converter control system with both fault current limiting and transient stability improvement, comprising a memory, a processor, and a computer program stored in the memory; characterized in that: The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 3.

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